## inkyvoyd 2 years ago Related rates (parametric derivatives)

1. inkyvoyd

if y=3x^2+2x and dx/dt=4, find dy/dt when x=3 My answer: dy/dt=24x+8; dy/dt=80. Can someone please check this? THanks!

2. tamtoan

isn't it dy/dt = 6x dx/dt + 2dx/dt ? can someone check this ?

3. inkyvoyd

:S

4. zepdrix

Yah @tamtoan is on the right track there,$\huge y=3x^2+2x$If we take the derivative with respect to TIME, we'll get,$\huge \color{#CC0033}{\frac{dy}{dt}}=6x\color{#F35633}{\frac{dx}{dt}}+2\color{#F35633}{\frac{dx}{dt}}$And they gave us information about dx/dt and x.$\huge \color{#CC0033}{\frac{dy}{dt}}=6(3)(\color{#F35633}{4})+2(\color{#F35633}{4})$Understand where those numbers are coming from?

5. inkyvoyd

Uh, the answer is defienitely 80, because I just submitted te assignment. Let me go over what happened though

6. inkyvoyd

Yeah, @zepdrix , you have the dx/dt because x is a function of t; @tamtoan didn't have that. Thanks.

7. zepdrix

Tam had the same thing, he just didn't plug in the x and dx/dt values for you :D

8. inkyvoyd

Oh he did :S. Well, thanks for helping out!